In the initial stage of semiconductor industries, bubbling was in wide use where a carrier gas such as oxygen or the like was passed through water in a bubblier. Although this technique was advantageous in that a wide range of a moisture content could be covered, a problem on pollution could not be avoided, and thus, the technique is rarely used at present. Accordingly, an oxygen and hydrogen combustion method, i.e. a pyrogenic system, has been widespread in order to avoid the disadvantage of the bubbler.
(Disclosure of Prior Art Literature, etc.)
With regard to an improvement in thermal oxidation and a moisture generation method thereto, to which the invention is directed, the following prior art techniques are known.
(1) In Japanese Patent Laid-open No. Hei 6-163517 of Ohmi, there is described a low temperature oxidation technique of lowering temperatures in a semiconductor process. In Example 1 of this application, there is set out a method wherein hydrogen is added to a gas atmosphere comprising about 99% of argon and about 1% of oxygen in an amount of from 100 ppm to 1%, from which steam is obtained at a hydrogen combustion temperature of 700° C. or below, particularly, at 450° C. or below, by the action of a stainless steel catalyst. Moreover, in Example 2 of the application, it is stated to thermally oxidize silicon in an atmosphere consisting of 990% of oxygen and 1% of steam formed by use of a catalyst at normal pressures or under pressure at an oxidation temperature of 600° C.
(2) Japanese Patent Laid-open No. Hei 7-321102 (Yosikoshi) describes high temperature thermal oxidation on silicon surfaces at an oxidation temperature of 850° C. at a very low moisture concentration, i.e. 0.5 ppm of a very super low moisture content region or in a dry region, in order to avoid various problems ascribed to moisture.
(3) In Japanese Patent Laid-open No. Sho 60-107840 of Honma et al, there is described a thermal oxidation method of silicon wherein in order to reduce variations in moisture content caused by moisture in a dry oxidation environment, a very small content of moisture at a level of about several tens of ppm formed according to a conventional method is purposely added.
(4) Japanese Patent Laid-open No. 5-152282 (Ohmi I) discloses a thermal oxidation apparatus which has a hydrogen feed pipe whose inner surfaces are constituted of Ni (nickel) or a Ni-containing material in order to prevent the generation of particles from the tip of a quartz tube as set out hereinabove, and also has means for heating the hydrogen gas feed pipe. In this thermal oxidation apparatus, water is formed by bringing hydrogen into contact with Ni (or the Ni-containing material) inside the hydrogen gas feed pipe heat to 300° C. or over, and reacting the hydrogen activated species with oxygen or (an oxygen-containing gas) More particularly, water is formed according to a catalytic system involving no combustion, so that there is no possibility that the hydrogen feed pipe melts at its tip end to cause particles to be generated.
(5) Japanese Patent Laid-open No. Hei 6-115903 (Ohmi II) discloses a moisture generating method using a catalyst system which comprises the mixed gas-preparing step of mixing oxygen, hydrogen and an inert gas to prepare a first mixed gas, and the moisture-generating step wherein the first mixed gas is introduced into a reactor tube constituted of a material, which has the catalytic action and is capable of conversion of hydrogen and oxygen into radicals and the reactor tube is heated to cause the hydrogen and oxygen present in the first mixed gas to be reacted thereby causing water to be generated.
According to this method, a catalytic material, with which the reaction is able to proceed at lower temperatures, is used as the reaction tube for reaction between hydrogen and oxygen. Eventually, generation of water is enabled at low temperatures. Accordingly, where the mixed gas of hydrogen, oxygen and an inert gas is fed to a heated reaction tube, hydrogen and oxygen undergo complete reaction therebetween in the reaction tube at a temperature of 500° C. or below. Thus, a gas containing moisture can be obtained at temperatures lower than that of a combustion system.
Moreover, if a metal material alone is used for a gas contact portion after exclusion of all plastic materials therefrom and the metal surfaces are subjected passivation treatment, gases (moisture, hydrocarbons and the like) released from the surfaces become very small in amount. This permits more purified moisture to be generated in higher accuracy in a wide range of concentration (covering ppb to %). The passivation treatment is performed by thermally treating a stainless steel, which has been subjected to electrolytic polishing or electrolytic composite polishing, in an acid or weakly acidic atmosphere with an impurity concentration of several ppb or below.
(6) Japanese Patent Laid-open No. Hei 5-141871 (Ohmi III) discloses a thermal treatment apparatus which includes, as least, an opening capable of opening and closing it, through which an article to be treated is carried out and in, a furnace core tube having a gas supply port through which a gas is supplied thereinto, a heating means for heating the inside of the furnace core tube, a gas supply tube connected in communication with the gas supply port, and heating means for heating the gas supply pipe wherein at least inner surfaces of the gas supply pipe is made of Ni (or a Ni-containing material).
This thermal oxidation apparatus is provided with a hydrogen activated species-generating means for forming hydrogen activated species from a hydrogen gas or hydrogen-containing gas without involving generation of a plasma, which is located upstream of a position of an article to be treated which is placed inside the furnace core tube. A hydrogen gas or hydrogen-containing gas is introduced into the hydrogen activated species-generating means to generate activated species of hydrogen. To this end, if a silicon substrate formed with an oxide film thereon is, for example, placed in the furnace core tube as an article to be treated, the activated species of hydrogen diffuse into the oxide film and contributes to termination of dangling bond in the oxide film and at the interface of the oxide film/silicon. Thus, it can be expected to obtain a gate oxide film of high reliability.
(7) In Japanese Patent Laid-open No. Hei 5-144804 of Nakamura et al, there is set forth a technique of thermal treatment of a silicon oxide film with activated species of hydrogen formed by use of a nickel catalyst.
(8) At pages 128 to 133 of the Lecture Papers at the 45th Symposium of the Semiconductor Integrated Circuit Techniques promoted by the Committee of Electronic Materials of the Association of Electrochemistry, there is reported a silicon oxidation process in a strongly reductive atmosphere mainly comprising hydrogen radicals produced by use of a catalyst for application to a tunnel oxide film of flash memories and hydrogen from moisture.
(9) In Japanese Patent Laid-open No. Hei 6-120206 of Ohmi, there is described a sintering technique using hydrogen activated species which are produced by means of a nickel catalyst for an insulating film insulating and isolating a selective epitaxial growth region therewith.
(10) In Japanese Patent Laid-open No. Sho 59-132136 of Kobayashi et al, there is set out a process of oxidizing and reducing silicon and a refractory metal in an oxidative and reductive mixed atmosphere of moisture and hydrogen generated by an ordinary method.
Disclosure of the Invention
(Discussion on Prior Art and the Invention)
In the most recent MOS devices, which are fabricated according to a deep submicron design rule, it is required to form a gate oxide film, which is very thin at 10 nm or below, in order to keep electric characteristics of the finely divided elements. For instance, where a gate length is at 0.35 μm, a required thickness of the gate oxide film is at approximately 9 nm. If the gate length is at 0.25 μm, it is assumed that the oxide film thickness becomes so thin as to be at approximately 4 nm.
In general, a thermal oxidation film is formed in a dry oxygen atmosphere. Where a gate oxide film is formed, it has been conventional to use a wet oxidation process (usually at a ratio in partial pressure of moisture of several tens of %) for the reason that the density of defects in the film can be reduced. According to the wet oxidation process, moisture is formed as a result of the combustion of hydrogen in an atmosphere of oxygen, and the moisture is supplied to the surface of a semiconductor wafer (e.g. a wafer for making an integrated circuit or a mere integrated circuit wafer) along with oxygen, thereby forming an oxide film. In view of burning of hydrogen, hydrogen is ignited after oxygen has been sufficiently passed beforehand in order to avoid the danger of explosion. Additionally, the concentration of moisture in a mixed gas of hydrogen+oxygen serving as oxidation species is increased to a level of about 40% (a partial pressure of moisture occupied in a total pressure in the atmosphere).
However, it is indicated that the above combustion system has the problem: since hydrogen is ignited and burnt while being injected from a quartz nozzle attached at the tip of a hydrogen gas supply pipe, the resultant flame comes too near to the nozzle under conditions where the amount of hydrogen is too small; and the nozzle eventually melts by application of heat thereto to cause particles to be generated, which serve as a pollution source of a semiconductor wafer (on the contrary, if the amount of hydrogen is increased in excess, the resultant flame arrives at an end portion of the combustion tube, so that the quartz walls are caused to be melted, thereby generating particles, or the flame is cooled at the wall surfaces and may be put out, thereby presenting a problem on safety). Moreover, in the combustion system, the moisture concentration in the water+oxygen mixed gas serving as oxidation species is so high that hydrogen and a OH group are taken in the gate oxide film. As a result, structural defects such as of an Si—H bond, an Si—OH bond and the like are liable to be produced in the thin film or at the interface with a silicon substrate. These bonds are broken down by application of a voltage stress, such as hot carrier injection, to form a charge trap, thereby causing electric characteristic of the film such as a variation in threshold voltage to be lowered.
It will be noted that the details of these situations and the details in an improved water-forming device using a novel catalyst are described in Japanese Patent Laid-open No. Hei 9-172011 of the present inventors and International Patent Laid-open No. PCT/JP97/00188 (international filing date: Jan. 27, 1997) of the present inventors and Ohimi et al.
According to the studies made by us, known oxidation formation methods are difficult in forming a very thin gate oxide film of a high quality and with a thickness of 5 nm or below (although it is as a matter of fact that similar effects can be expected when the thickness is 5 nm or over) in a uniform thickness and in high fidelity. Needless to say, the formation of a thicker film is also unsatisfactory in many respects.
In order to form a very thin oxide film in a uniform thickness in high fidelity, it is necessary to form a film at an oxide film growing rate lower than that for the formation of a relatively thick oxide film and under more stable oxidation conditions. For instance, in the oxidation film formation method using such a combustion system as set out before, the moisture concentration in a water+oxygen mixed gas serving as oxidation species can be controlled only within a range of concentration as high as from 18% to about 40%. Under these conditions, the oxidation film growth rate is so high that with a thin oxide film, the film can be formed within a very short time. On the other hand, if the oxidation is carried out at a wafer temperature of 800° C. or below in order to lower the film growth rate, the film quality lowers (although the present invention is, of course, applicable to in a temperature range of 800° C. or below by appropriately controlling other parameters).
For the formation of a clean oxide film, it is necessary to remove a low-quality oxide film formed on the surface of a semiconductor wafer by wet cleaning beforehand. However, a thin natural oxide film is inevitably formed on the wafer surface on the way of transferring the wafer from the wet cleaning step to an oxidation step. Moreover, in the oxidation step, an undesirable initial oxide film is formed on the wafer surface by contact with oxygen in the oxidation species prior to intended oxidation. Especially, with the oxide film formation method using a combustion system, hydrogen is burnt after sufficient flow of oxygen in order to avoid the danger of explosion of hydrogen, so that a time of the wafer surface being exposed to oxygen is prolonged, thereby forming a thick initial oxide film (it is accepted that explosive combustion of hydrogen, i.e. “explosion”, takes place under conditions of normal pressures, a temperature of 560° C. or over, a hydrogen content of 4% of over, and a sufficient content of oxygen).
In this way, an actual oxide film has an arrangement that includes, aside from an oxide film formed by intended oxidation, a natural oxide film and an initial oxide film. These natural oxide film and initial oxide film are both lower in quality than the intended, intrinsic oxide film. Accordingly, in order to obtain a high-quality oxide film, it is necessary to suppress the ratio of the lower-quality films to the total oxide film to a level as low as possible. Nevertheless, when a very thin oxide film is formed according to known oxide film formation methods, the ratio of these lower-quality films rather increases.
For example, when a 9 nm thick oxide film is formed using a known oxide film formation method wherein the thicknesses of a natural oxide film and an initial oxide film in the oxide film are assumed to be at 0.7 nm and 0.8 nm, respectively, the thickness of the intrinsic oxide film is at 9−(0.7+0.8)=7.5 nm. The ratio of the intrinsic oxide film in the total oxide film is at about 83.3%. However, when a 4 nm thick oxide film is formed according to the known oxide film formation method, the thicknesses of a natural oxide film and an initial oxide film are not changed at 0.7 nm and 0.8 nm, respectively, the intrinsic oxide film thickness is thus at 4−(0.7+0.8)=2.5 nm, with its ratio being lowered to 62.5%. More particularly, if a very thin film is formed according to the known oxide film formation method, not only uniformity and fidelity of a film thickness are not assured, but also the quality of the film lowers.
In order to solve these problems, we made attention to moisture generation methods of Ohmi et al using catalysts. According to our studies, these methods place emphasis on the strong reduction action of hydrogen radicals on the assumption that “the life of hydrogen radicals is long”. Therefore, it will be apparent that these methods cannot be applied to a mass-production process of semiconductor integrated circuits as they are. In other words, for application to a semiconductor process, we have found that necessary parameters have to be studied on the assumption that “the life of hydrogen radicals is very short, and the radicals are generated on a catalyst and are chemically combined or returned to a ground state thereon or in the vicinity of the catalyst”.
Further, according to the present inventors, it has been made clear that a ratio of partial pressure of moisture ranging from 0 to 10 ppm belongs to a dry region wherein a nature of so-called dry oxidation appears, which is inferior to so-called wet oxidation with respect to the film quality required for a gate oxide film and the like in a fine process described hereinafter.
Likewise, we have found that a super low moisture region, wherein a partial pressure ratio of moisture ranges from 10 ppm to 1.0×103 ppm (i.e. 0.1%), principally exhibit a nature substantially same as dry oxidation.
Moreover, it has also been found that thermal oxidation in a low moisture region covering a moisture partial pressure ratio in the range of form 0.1% to 10% (especially, in a low moisture region covering a moisture partial pressure ration in the range of 0.5% to 5%) is relatively better in properties than those in other regions (including a dry region, a region ordinarily employed in a combustion system of 10% or over, and a high moisture region having a moisture concentration of several tens of % attained by use of a bubbler or the like)
(Objects, etc. of the Invention)
An object of the invention is to provide a technique wherein a high-quality very thin oxide film is formed in a uniform thickness and in high fidelity.
The above and other objects and novel features of the invention will become apparent from the description of the present specification and the accompanied drawings.